Current measuring circuits have many applications and, in particular, one such application is in a trip unit for a circuit breaker providing ground fault protection. U.S. Pat. No. 4,631,625, issued on Dec. 23, 1986, discloses a microprocessor-based trip unit for a circuit breaker within which a current measuring circuit is incorporated. In U.S. Pat. No. 4,631,625 the current measuring circuit is generally illustrated as including current transformers, a rectifier, signal converters and an analog inverter. The microcomputer of U.S. Pat. No. 4,631,625 ultimately utilizes the signals from the analog inverter to calculate a ground fault current. U.S. Pat. No. 4,947,126, issued on Aug. 7, 1990, also discloses a ground fault protection circuit for monitoring multi-phase alternating current signals and for providing a set of positive (full wave rectified, direct current) signals each associated with the alternating current signals being monitored.
Some ground fault protection schemes modify signals representative of the alternating current being measured in such a way that neither the phase nor the polarity information of these signals is preserved. A Root Mean Square (RMS) analysis of these signals is then performed without the polarity information in order to calculate a ground fault current. U.S. Pat. No. 4,927,126 discloses an improved apparatus for monitoring an alternating current signal that preserves the collective polarity information of the current signal, partly improving the accuracy of ground fault current calculations. However, the apparatus does not independently recover phase information for each phase of a multi-phase alternating current signal being monitored.
Accordingly, it would be advantageous to develop a further improved current measuring circuit for an alternating current signal that allows the subsequent complete and independent reconstruction of the signal with the same fidelity of the original signal (i.e. with phase and polarity information as well as magnitude information preserved), to improve the accuracy of ground fault current calculations or other calculations relying upon the reconstructed signal.